[Devel] Re: [ckrm-tech] [RFC][PATCH][2/4] Add RSS accounting and control

[Andrew Morton]

On Mon, 19 Feb 2007 16:07:44 +0530 Balbir Singh <balbir at in.ibm.com> wrote:

> >> +void memctlr_mm_free(struct mm_struct *mm)
> >> +{
> >> +	kfree(mm->counter);
> >> +}
> >> +
> >> +static inline void memctlr_mm_assign_container_direct(struct mm_struct *mm,
> >> +							struct container *cont)
> >> +{
> >> +	write_lock(&mm->container_lock);
> >> +	mm->container = cont;
> >> +	write_unlock(&mm->container_lock);
> >> +}
> > 
> > More weird locking here.
> > 
> 
> The container field of the mm_struct is protected by a read write spin lock.

That doesn't mean anything to me.

What would go wrong if the above locking was simply removed?  And how does
the locking prevent that fault?


> >> +void memctlr_mm_assign_container(struct mm_struct *mm, struct task_struct *p)
> >> +{
> >> +	struct container *cont = task_container(p, &memctlr_subsys);
> >> +	struct memctlr *mem = memctlr_from_cont(cont);
> >> +
> >> +	BUG_ON(!mem);
> >> +	write_lock(&mm->container_lock);
> >> +	mm->container = cont;
> >> +	write_unlock(&mm->container_lock);
> >> +}
> > 
> > And here.
> 
> Ditto.

ditto ;)

> > 
> >> +/*
> >> + * Update the rss usage counters for the mm_struct and the container it belongs
> >> + * to. We do not fail rss for pages shared during fork (see copy_one_pte()).
> >> + */
> >> +int memctlr_update_rss(struct mm_struct *mm, int count, bool check)
> >> +{
> >> +	int ret = 1;
> >> +	struct container *cont;
> >> +	long usage, limit;
> >> +	struct memctlr *mem;
> >> +
> >> +	read_lock(&mm->container_lock);
> >> +	cont = mm->container;
> >> +	read_unlock(&mm->container_lock);
> >> +
> >> +	if (!cont)
> >> +		goto done;
> > 
> > And here.  I mean, if there was a reason for taking the lock around that
> > read, then testing `cont' outside the lock just invalidated that reason.
> > 
> 
> We took a consistent snapshot of cont. It cannot change outside the lock,
> we check the value outside. I am sure I missed something.

If it cannot change outside the lock then we don't need to take the lock!

> 
> MEMCTLR_DONT_CHECK_LIMIT exists for the following reasons
> 
> 1. Pages are shared during fork, fork() is not failed at that point
>     since the pages are shared anyway, we allow the RSS limit to be
>     exceeded.
> 2. When ZERO_PAGE is added, we don't check for limits (zeromap_pte_range).
> 3. On reducing RSS (passing -1 as the value)

OK, that might make a nice comment somewhere (if it's not already there).